This invention concerns a high frequency electrosurgical apparatus for coagulation of biological tissue of a patient, of the kind comprising a high frequency generator of electrical alternating current and voltage, equipped with switching arrangements for manual or automatic activation of the high frequency current and for automatic termination of the coagulation operation as soon as it has reached a certain stage.
Thermal coagulation of biologic tissues, hereinafter called coagulation, by means of high frequency alternating electrical current and voltage, hereinafter simply called high frequency current, has been used for over 50 years in human and veterinary medicine for devitalizing diseased tissues and for closing perforated blood vessels. To this end, the high frequency current is conducted through the tissue to be coagulated so that the heating of the tissue experiencing the current takes place endogenically. The heating of the tissue is dependent on various parameters, such as the amount of tissue to be coagulated, the intensity of the high frequency current, the specific electrical resistance of the tissue to be coagulated, the duration of flow of the high frequency current, the specific heat capacity of the tissue to be coagulated, the shape and size of the coagulation electrodes, the heat dissipation through the coagulation electrodes, and also on whether the coagulation is performed in a monopolar or bipolar manner.
Since these parameters vary to a more or less great degree both during a coagulation process and from one coagulation process to another, the replicability of a desired quality of coagulation is very difficult to obtain and requires great attention and skill on the part of the operator.
In the various fields of surgery, various coagulation techniques have been developed, requiring satisfactory properties in the equipment used for the coagulation. As a rule, in nearly all surgical fields, conventional high frequency surgical devices are used for the different coagulations that even today have only adjusting devices for the intensity of the high frequency current and switches for switching the high frequency current on and off. And when these conventional high frequency surgical devices are used, the operator adjusts the intensity of the high frequency current at the adjusting device in accordance with his experience and allows the high frequency current to flow through the tissue to be coagulated until such time as he has the impression that the coagulation is completed.
Since as a rule the coagulation processes elapse within only 0.5 to 5 seconds, and changes in the coagulating tissue can become perceptible to the operator only within a fraction of these relatively short coagulation times, it is in practical terms almost impossible to terminate the coagulation process manually at precisely the time at which the coagulation is optimal. If the operator sets the intensity of the high frequency current too low and/or switches off the high frequency current too early, then the coagulation temperature is not attained and coagulation is inadequate. If the operator sets the intensity of the high frequency current too high and/or switches the high frequency current off too late, then the temperature of the coagulate rapidly increases beyond the required coagulation temperature, to above 100.degree. C., which can very rapidly cause endogenic vapor formation within the coagulate and hence entail the explosive collapse of the coagulate, so that the intended goal of coagulation, especially when the coagulation is used for closing perforated blood vessels, is not attained.
Optimal coagulations are particularly difficulty to attain if the operator is not able to observe to coagulation process, for example if the coagulation has to be performed by means of invasive coagulation electrodes, such as in stereotactic operations in the neurosurgical field, or in tranvenous ablation of an accessory pathway in therapy-resistant supraventricular reentry tachycardias, as well as in transabdominal tubal ligation.
Attempts have been made for many years to devise an apparatus which automatically terminates the coagulation process as soon as the coagulation has attained a defined stage.
From German Patent Disclosure Document DE-OS No. 31 20 102, an apparatus for high frequency coagulation of albumen for surgical purposes, which is intended to shut off the high frequency current at the instant when the impedance or electrical resistance of the coagulate progresses through a certain change during the coagulation process.
It has been known for many years that the specific electrical conductance of electrolyte-containing biologic tissues largely increases in proportion to the temperature of this tissue. However, as soon as the temperature at which coagulatable components of the tissue coagulate is reached, which is the case between approximately 50.degree. C. and 80.degree. C., the increase in the electrical conductance becomes progressively less as a function of the temperature. Above 80.degree. C. and in particular in the vicinity of the boiling point of intracellular and extracellular fluids, the electrical conductivity decreases rapidly. The cause of this rapid decrease in electrical conductivity above 80.degree. C. is suspected to be both endogenous vapor formation and the dessication or drying up of the coagulate.
In the apparatus for high frequency coagulation of albumen, in particular for surgical purposes, described in German Patent Disclosure Document DE-OS No. 31 20 102, the albumen impedance is determined continuously while high frequency power is being supplied and the differential quotient for the impedance curve is formed as a function of time. The values of the differential quotient serve first to set the initial power, and second to determine the optimal instant for shutoff of the high frequency power. For both situations, pre-adjustments are provided that make for a flexible and simply adaptable apparatus. The apparatus is intended preferably for bipolar coagulation instruments, but use with unipolar instruments is also possible.
The use of this apparatus is problematical because the changes in impedance of the coagulating tissue are not continuous, as a result of fluctuations in the contact pressure between the coagulation electrode and the tissue caused by trembling of the hand of the operator and by shrinkage of the coagulating tissue, and instead more or less fluctuate about a mean trend. As a result, there are many undefined zero positions of the differential quotient. Furthermore, the differential quotient during slowly elapsing coagulations, because of the slow changes in the albumen impedance, is so small that a defined shutoff time does not exist.
It is an object of the invention to provide an apparatus for the thermal coagulation of biologic tissue by high frequency electric current in which the coagulation operation is controlled in such a way that better operation results are attainable while at the same time the surgeon is relieved of the responsibility of manually measuring out short intervals of time, and instead the coagulation operation can be automatically terminated at a precisely defined condition of coagulation.